In a simple information terminal not provided with an auxiliary memory device, control software is stored in a ROM that is not rewritable, and to update this control software, the ROM must be replaced, which is very difficult if multiple products are present in the market. For flexible changing of control software, the so-called "program downloading system" is known, in which system the control software is stored in a RAM instead of the ROM and transmtted from the central processing apparatus of the system through a transmission channel. In this system, even if it is not necessary to change the software, transmission is always required when turning on the power source or when recovering from abnormal state of the terminal. To eliminate such wasteful transmission when turning on the power source, a method for storing the control software in the EEPROM instead of the RAM is proposed (for example, "Program downloading system" in Japanese Laid-open Patent No. 6-4269). In this method, it is impossible to change the download processing portion of the control software.
It is also proposed to prepare two sets of EEPROM, and download the control software for updating in a spare EEPROM while managing by the working EEPROM ("Remote software downloading system" in Japanese Laid-open Patent No. 5-158703). According to this system, if an abnormality occurs while downloading, since the working control software remains intact, it is a benefit that downloading into the spare EEPROM can be resumed, and another merit is that the download processing portion of the control software can be changed. A demerit is, however, that the cost is high because two sets of EEPROM are used.
Further, concerning the digital television receiver, in order to cope with extension of functions flexibly, it is proposed to store the basic function portions of the control software in the ROM, and store the extension portions of the control software corresponding to the extended functions in the EEPROM ("Data reception processing apparatus, data reception processing method, and broadcasting method" in Japanese Laid-open Patent No. 8-195952). Branching means to a predicted extension function is prepared permanently in the basic function portions of the control software stored in the ROM, and the extended portions of the control software are downloaded in the EEPROM by using the data transmission stream contained in the digital broadcasting channel. This system is advantageous in that it is flexible to cope with predicted extensions, but as for the extension not predicted, it is necessary to change the basic functional portions stored in the ROM, which requires replacement of ROM, and hence it is not a radical solution.